Generally, characteristics of the transmitted pulse are linked with a particular imaging mode. For example, the duration of the pulse is adjusted depending on whether the scanner operates in B-Mode, Pulsed Wave (PW) Doppler, or Color Flow Imaging. Likewise, the center frequency of the pulse is set according to the frequency response of a transducer.
To improve the propagation characteristics of the sound wave, the interrogating pulse of many medical ultrasonic imaging systems is constituted as a carrier wave modulated by a gradually rising and gradually falling amplitude envelope. In some cases, the envelope of the pulse is Gaussian in shape. Its advantage is that an initially Gaussian pulse retains its Gaussian shape during propagation through an attenuating media such as tissue.
One more reason for shaping transmit pulses has come in conjunction with the harmonic imaging where it is particularly valuable to reduce the level of even harmonics.
Summarizing, both fundamental and sub-harmonic mode ultrasound imaging are all improved by controlling the bandwidth of the frequency spectrum of the transmitted ultrasonic pulse. This bandwidth is preferably limited to frequencies around the fundamental, and is preferably sharply reduced at specified harmonics and sub-harmonics of the fundamental.
Referring to the transmit techniques, there are two main classes of ultrasound transmitters: pulsers and so-called arbitrary waveform generators (AWG).
Arbitrary waveform generators have been advanced solution for high-end ultrasound systems (by way of example, see U.S. Pat. Nos. 5,549,111, 5,970,025, 6,104,673, and 6,469,957). AWGs can provide instantaneous change in transmit energy between transmit pulses, apodization profiles with greater resolution, and acoustic beams with lower harmonic content. Transmit signals produced by an AWG are typically Gaussian or Hamming modulated cosines individually formed for each transducer element. In operation, AWGs use stored digital representations of shaped waveforms, comprising a digital-to-analog converter and a power amplifier (see U.S. Pat. No. 6,537,216) to produce an analog drive signal for the transducer. However, the current implementation of arbitrary waveform transmitters is limited because of its high manufacturing cost, power dissipation and space constraints.
A typical excitation signal produced by a pulser looks like a gated square wave of a desired carrier frequency. The waveform shape or envelope is essentially fixed, and, therefore, not optimal. The only adjustable parameter of a basic pulser is the length of the gate in terms of an integer number of carrier cycles. Advanced pulsers use the pulsewidth modulation (PWM) techniques and operate as described below.
To generate the PWM transmit waveform, the modulating signal is compared with a high frequency sawtooth or triangle waveform that acts as a carrier. The resulting binary signal of the comparator feeds a suitable set of power switches connected to the power supplies. Having low power dissipation, PWM (or class-D) pulsers can be effectively integrated. For the same reason, the PWM pulsers are more preferable in terms of heatsinking. These factors and the relatively low cost of such ultrasound transmit circuits are the forces behind the motivation of their widespread use.
By way of example, U.S. Pat. No. 6,135,963 entitled “Imaging System with Transmit Apodization Using Pulse-Width Variation” describes a method and apparatus for transmit apodization by controlling the duty cycle of the pulse.
U.S. Pat. No. 5,833,614 and No. 6,432,055 discuss several types of PWM transmit waveforms that can be used to approximate a carrier wave modulated by a gradually rising and gradually falling amplitude envelope. Such PWM waveforms include various unipolar sequences having two voltage levels (+V, 0) and bipolar sequences having three voltage levels (+1V, 0, and −1V).
Ultrasound equipment with harmonic imaging capabilities transmits a signal at one frequency and receives the echoes at twice that frequency. A substantial obstacle to using second harmonic imaging with PWM is minimizing transmit energy at the second harmonic frequency. One solution is based on selecting the number of cycles of a carrier frequency that are transmitted in a square wave pulse burst. A greater number of carrier cycles in the burst corresponds to a narrower signal bandwidth. However, any increase in the number of carrier cycles results in a lower range resolution of the system.
Pulse inversion harmonic imaging exploits two pulse bursts with the second burst as an inverse replica of the first one. This routine cancels even order distortion products that are capable of masking a valuable harmonic information generated by tissue. However, the alternation halves the image update rate. On the contrary, operating at higher frame rates is particularly important for cardiac imaging.
Yet another approach is to use a bipolar (3-state) uniform square wave pulse train, which has less energy around the second harmonic frequency than does a unipolar uniform square wave pulse train of the same length.
To suppress energy at second order harmonics further, U.S. Pat. No. 5,833,614 teaches the transmit waveform comprising pairs of identical pulses. The width of the pulses within each pair is modulated as a function of the envelope amplitude. These pulses are phase delayed by 90 degrees relative to each other. However, increasing the number of alteration in a waveform results in higher sampling rate and, generally, a more complex transmit beamformer. At the same time, while the second harmonic is substantially suppressed, the intensity of the odd harmonics is noticeable. For instance, referring to U.S. Pat. No. 5,833,614, the third harmonic level is approximately −4 dB.
An alternative 3-state, pulse width modulated, bipolar waveform is disclosed by U.S. Pat. No. 6,432,055. This pulse train is constructed by summing a first component with an inverted, time-shifted version of the first component. As shown in U.S. Pat. No. 6,432,055, by properly selecting the time interval for the time shift of the second component, filtering of the second harmonic can be obtained. At the same time, the filtered pulse spectrum is broadband including high-order harmonics and sub-harmonics of the fundamental, each of a considerable intensity.
Therefore, there is still a need for improved transmit sequences that are band-limited and suppress energy at selected harmonic frequencies, and for transmit generators that are capable of generating such sequences.